Heat 2 T. of the olive oil in a saute pan or skillet over medium-high heat. Add the onion and saute until soft but not browned, about 5 minutes. Add the garlic and saute 1 minute. Add the spinach and saute until the spinach turns bright green, about 3 minutes. Transfer to a mixing bowl. Stir in the ground beef, Parmesan cheese, bread crumbs, parsley, egg, nutmeg, salt, and pepper to taste.

Quickly rinse the beef under cold running water and pat dry with paper towels. Lay the meat on a flat surface and use a sharp knife to butterfly it by slicing horizontally, leaving one edge intact. Spread the steak out and gently pound with a wooden mallet or flat side of a meat cleaver to an even thickness. Layer the mortadella slices on top of the meat, then spread with the spinach mixture. Sprinkle the peas over the spinach. Beginning at one long side, roll the meat up jelly-roll fashion, tucking in ends, and tie securely in several places with cotton string.

Heat the remaining 3 T. olive oil in a dutch oven or large pot over high heat. Brown the beef roll on all sides. Stir the tomato paste, wine, and stock together in a bowl and pour over the meat. Bring the liquid to a boil. Cover, reduce the heat to low, and simmer until the beef is tender when pierced with a fork, about 1-1/2 to 2 hours, turning several times and adding more stock, broth, or water if necessary to maintain about 1 cup liquid.

Remove the beef to a cutting board and let it stand about 10 minutes. Meanwhile, place the pan over high heat and reduce the sauce if it seems too thin. Remove the strings and carve the roll into 1/4-inch slices. Season the hot pan juices with salt and pepper to taste and spoon over the meat.